The Oracle DB currently ships with, uses AND MUST USE Java 1.5. You cannot update the DB version used internally and you cannot disable it without breaking the DB. So don't try to import code that requires features only found in later Java versions.

If you attempt to upgrade the DB Java version you will likely break Oracle in a way that cannot be recovered without reinstalling the database. That operation is NOT supported and will NOT be successful.

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So far I did not intend to do this but at least I was told it would be possible.
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No problem - it's just that trying to remove or update the Oracle JVM can be catastrophic.

Oracle itself uses the JVM for many, many things even if the users do not.

Just for reference here is one of the explicit statements from one of the docs: Oracle® Database Installation Guide 11g Release 1 (11.1) for Linux under section 6.8 Removing Oracle Software
http://docs.oracle.com/cd/B28359_01/install.111/b32002/remove_oracle_sw.htm
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Note:

If you choose to remove Oracle JVM, Oracle Universal Installer removes all installed products that depend on Oracle JVM, including Oracle Database 11g.